In case of radiation treatment using small field high-energy photon beams, an accurate dosimetry is a challenging task because of dosimetrically unfavorable phenomena such as dramatic changes of the dose at the field boundaries, dis-equilibrium of the electrons, and non-uniformity between the detector and the phantom materials. In this study, the absorbed dose in the phantom was measured by using an ion chamber and a diode detector widely used in clinics. GAFCHROMIC^Ⱂ EBT films composed of water equivalent materials was also evaluated as a small field detector and compared with ionchamber and diode detectors. The output factors at 10 cm depth of a solid phantom located 100 cm from the 6 MV linear accelerator (Varian, 6 EX) source were measured for 6 field sizes (5×5 cm², 2×2 cm², 1.5×1.5 cm², 1×1 cm², 0.7×0.7 cm² and 0.5×0.5 cm²). As a result, from 5×5 cm² to 1.5×1.5 cm² field sizes, absorbed doses from three detectors were accurately identified within 1%. Wheres, the ion chamber underestimated dose compared to other detectors in the field sizes less than 1×1 cm². In order to correct the observed underestimation, a convolution method was employed to eliminate the volume averaging effect of an ion chamber. Finally, in 1×1 cm² field the absorbed dose with a diode detector was about 3% higher than that with the EBT film while the dose with the ion chamber after volume correction was 1% lower. For 0.5×0.5 cm² field, the dose with the diode detector was 1% larger than that with the EBT film while dose with volume corrected ionization chamber was 7% lower. In conclusion, the possiblity of GAFCHROMIC^Ⱂ EBT film as an small field dosimeter was tested and further investigation will be proceed using Monte Calro simulation.

KeywordsSmall field, Absorbed dose, GAFCHROMIC^Ⱂ EBT, Ion chamber, Diode detector